Fuel feed control for gas turbine engines



Feb. 9, 1954 H, oEsTRlcH ET AL FUEL FEED CONTROL FOR @As TURBINE ENGINES Filed July 27 Patented Feb. 9, A1954 FUEL FEED CONTROL FOR GAS TURBINE ENGINES Hermann Destrich, Siegfried Decher, and Wolfgang Stein, Decize, France; said Decher and said Stein assignors to said Oestrich Application July 27, 15.948, Serial No. 40,954

`Claims priority, application France August 4, 1947 7 Claims said air and the International Standard Atmos- 1 phere indicated by the abbreviation I. N. A.

vIn order to obtain a maximum efficiency, and maximum life, for the turbine, such a regulation should be effected in order that the component parts of the engine which are stressed to a maximum extent at the wideopen position of the throttle are equally stressed irrespectively of the altitude and flight speed. Such a process of regulation will thus prevent any overstress for the engine component parts.

In order to solve this problem of regulation, it is desirable to keep constant, during full-throttle running, a given number of revolutions and gas temperature, whateverbe the conditions of flight which the plane may meet. In the known jet receiving the exhaust discharge from the engine.

In turbine driven propeller units, the regulation is effected by altering the propeller pitch, instead of adjusting the exhaust jet nozzle.l

A primary object of our invention is to provide engine control means which arerelatively simple and easily applicable to various types of jet engines.

This invention relates to means for the control of jet engines such as turbojet engines andpropjet engines, through which fuel is supplied to the engine by the required amount for securing a constant temperature in the combustion chamber and a constant R. P. M. for said engine at fullthrottle running, irrespectively of the actual conditions of air admitted to the engine.

Furthermore this invention relates to means for-the control of `jet engines such as turbojet engines and propjet engines, through which the required amount of fuel supplied to the engine is adjusted in dependence of the temperatureand f' According to a modification of the invention, the regulation of the amount of fuel sent tothe engine isv combined with a regulation of propulsion means as a function of the rotation speed of the engine in order to keep this rotation speed at a constant value.

According to another characteristic of the invention, the amount of fuel to be fed to the engine is regulated by determining the maximum amount of fuel for feeding this engine at fullthrottle running and in the atmospheric conditions of high speed, low altitude and great cold flights, supplyingr said amount to the fuel supply line for the engine, and causingonly that portion thereof which is actually required under the prevailing conditions, to be forwarded to the engine While withdrawing the remainder from said line.

The invention is also relative to an arrangement for carrying into practice the preceding, or similar processes, characterized by means for adjusting the amount of fuel supplied to the engine, which are controlled from a heat expansible device responsive to the temperature of vair at the intake for the compressor and a pressure deformable device responsive to the absolute pres-4l sure of said air. v 4

According to a feature of the invention, the fuel-system includes, before the fuel control Vdevice, manually operable means for adjusting the rate of flow therethrough to values between zero and the maximum amount of supply to the engine as above defined, according to the desired power output. f

The invention nally includes the features described below and their various possible combinations.

An arrangement of the engine control means according to the invention'is shown, as an examf ple, on the attached drawing, on which the lsole figure is a diagrammatic general view of the adjusting device according to the invention, adapted to a jet propulsion engine.

The structure of the control means according to the invention is based on the following considerations: v

The amount of fuel necessary, during the fullthrottle running and in order to keep constant the temperature in thecombustion chamber andA the number of revolutions dependson the vari-,J able characteristics of the air before the coinpressor. u y 4 With all turbojet engines and turboprop' eri-' y gines, if VB is the total amount of fuelsuIpplfedfA to the combustion engine, b is the amountof not very important. We have found that for practical purposes the amount of fuel supplied the engine may be considered as substantially corresponding to the formula:

=all+f 2s8-fn 1+b in which:

=theoretica1 amount of fuel fortliefllthrottle running when p1=1 atm.. and T1=-288| h r=temperature coeicient Tapproiimately constant for any engine).

isar'ne-*s'calebasia in the :sense'tlfiat for the"same 'va-lue--'\f'-normal`teiriperaturev T1: the'correspondingf'pint' onthe two ilines vshould yield-the same Value) it Will be seen that the corresponding cu'rve's :are -different, vvthecu'rVe corresponding to '('a constant -"T1)*bei`n`g appreciabl-y steeper "than `-the curve correspondingto v1/T1. I-Ience it 'may b'efsald thatithe influence of 'air'temp'eratur'e T1 on the amount of fuel v"1B-#b 'corresponding vto the above'f'ormulais :greater thanfif the 'amount ffffuelweresupplied vin proportiowto I1/T1. ,As a kfconsecj'u'e'nf'z'e we found it: impossible to ele'ct an automatic regulation of the fuel 'amount sup- 'pliedto'tlie'burner in dependence-'on air density witharv'iew todeliveringfuel in"accordance with our formula. 'We'fou'n'diit necessary 'toprovide separate f air temperature and Vairr responsive devices-and" toI provide-a Lsuitable llinkage for interconnecting-said devices' 'and vthe throttling member designed to control the fuel metering orifice.

:iWith control m'eans according-"to this inventio'n, *the amount 'fof fuel corresponding vvto the ideal conditions is fed into the combustion chamberfso thatgat fullthrottleythenumberof revolutions and the 'temperature of the Icombustion hamber *remain lconstant whatever the :flight conditions may be. The-measure of 'the'necessary-amountff fuel isI obtained here only'bytaking into account the absolutevpressure' andthe temperature of theair'sucked in at the intake' of therk compressor, 'these two 'quantities being u'sed asa basis for the adjustment of this'am'ount cf fuel.

Ac'coitling`to aform' of vthe invention,r the regu- Iationby'modifyingthe cross-'section of -the eX- ha'u's't nozzle is'g'ivenup,the`regulation being only obtained in modifying the amount f 'fuel r`fed ,to the injection jets and in distributing this ful according'.tb'the'amount"computed forthe ideal case.

In that case, it is desirable to design a safety device preventing the number of revolutions from exceeding a predetermined limit.

According to a variant of the invention, it will be possible to combine thefuel control means as labove referred to With knownifm'eans for adjusting the propelling device as a function of the R. P. M. of the engine so as to keep said R. P. M.

constant; a speed governor driven from the engine is employedfor that purpose.

,'In the case of a iixed exhaust nozzle, the struc- .ture of the fuel control means characterizing the invention is base'don the following facts:

Whenan amount of fuel corresponding to the ideal case is injected in a jet propulsion engine including -a -iixed exhaust nozzle, the required values for the number of revolutions and the gas temperature are obtained only if the cross-section of the .exhaust nozzle corresponds, by chance, to that which is suitable for the case considered, but if the cross-section ofthe exhaust-nozzle islarger vthan that of the optimum-nozzle, the numbeimof revolutions and the air supply lto the engine are superior to the `required values.

As, by'supposition, the amount of injected-.fuel remains the same, the result is a decreaseof the gas temperature.

When,'on the contrary, the fexhaustnozzlehas dimensions, and specially a.sectionof-passage inferior to thoseof the theoretical optimum nozzle, the rotation speed and. the air supply to-the engine are inferior to the theoretical valuesand the gas temperature Ais Asuperior to the theoretical value.

`In that case,v the thrust. givenby Lthe motor-is not very different from the theoretical vvalue-obtained with aconstant numberA of revolutionsand a constant gas temperature,because it.is..known that-in turbo-jet engines,vthe fuel consumption per unit of power remains.substantiallyconstant for changes vof regulatingconditions having'little importance. l

In other w'ords,in the-case where the: inner cross-sectionof the exhaust nozzleds fixedto the value required for averageflight conditions,-and the Aamount of injected fuel corresponds tothe ideal Vin each flight .conditionpa .positive deviation ofthe number of revolutions `of the engine will havev asl a consequence a negative deviation of the gas temperature and vice versa.

Consideringthat'moreoften than not theblading of the turbine is Ythe 4most-stressed part of the unit, it will-'be understood `that vthe Yfeature describediabove (to wit,.positivedeviationvof the R. P. M...corresponding `to ay negative deviation of gas temperature-:and-vice fversa) .for-a unit having a fixed exhaust nozzle and fuel supply control means in accordance withthis invention, is favourable `to a substantiallyinvariable stressing of -the rturloinebladessinceany increase of centrifugal forces is vcompensated for.by-afde crease of heat effectsand-conversely.

-In they case where .the exhaust nozzlesv are not adjustable, the unavoidable .variations .of number of revolutions 4of the-engineland of -the gas A`temperature can `generally be maintained within acceptable limits, these --variations being then ywithout notablednluence ion` the propulsion force developed: byrthe. jet propulsion engine.

`Consequently the-'control fmeans accordingnto the invention requireffor .being .carriedintopractice,'the following :essential-elements:

-In allcases,.a device for regulatingtheamount of.. fuel.

In the case of turboj et units, sometimesasafety l.device permittneto lirnfitgthe nur nber,y of revoluthe compressor, which can be caused to act on deformable evacuated capsules to effect control, A(b) The linear term relative to temperature.

vThisterm can be given by a temperature recorder sensitive to expansion.

It is the action of these two terms (a) and (b) which, after being multiplied, is transmitted to the device regulating the amount of fuel.

On the drawing is shown diagrammatically a regulating arrangement for carrying this invention into practice.

The turbojet unit is enclosed in a shell or casing shown at the top part of the figure, and the outline of the lower half of said unit is partly indicated by dotted lines; the main component parts of the unit comprise a compressor I, a combustion chamber 2 containing fuel injection nozzles or jets 22, a turbine 3 keyed to the same shaft as the compressor, and a non-adjustable exhaust nozzle 4.

According to the invention, this engine is cornbined with means for controlling the supply of fuel to nozzles 22 which include the following parts:

(a) Hand controlled fuel regulating device A fuel feed unit comprising pumps and a filter between the same, generally denoted 5, is provided to forward fuel through a conduit 61 to a valve 'I operated by hand by an operating lever` 9, a connecting rod I and a crank I I.

The valve 'l comprised for instance of a plug having a fuel passageway therethrough and rotating in a shell can feed the fuel, according to its position, either in a conduit 62 having at its inlet a measuring orifice 8, either in a by-pass I0 turning round this orifice 8 and provided with a flow control device such as a screw-threaded needle valve I0. This by-pass is used for running without load. l

The head loss of the fuel flow, produced when passing through orifice 8results in a difference of pressure between the parts of the conduit above and below the orifice 8. Interposed between the fuel delivery conduit 61 `from the fuel feed unit and a fuel conduit I6 is a valve casing I4 with a fuel supply branch pipe |51 from conduit 51 to said casing and a fuel discharge pipe |52 from casing I4 to return conduit I6. Slidably movable in casing I4 to control the ports therein communicating with pipes |51 and |52 is a piston 41, a spring I 42 being interposed between said piston |41 and casing I4. Pipes I2 and I3 are interposed between conduits 61, 62 respectively and the opposite end chambers in valve casing Ill as shown. As the output of fuel feed unit 5 is higher than that which will be adjusted by orifice 0, an increasing fuel pressure prevails above this orifice. When this pressure reaches a value high enough for overcoming the strength of spring |42 and thus moving piston |41 to the right, said pis-A ton allows for discharge of fuel into the exhaust conduit |52, which decreases the pressure above orifice 8. 1 s

1 Forfanygiven 1position 1 ofcvalve FI.; asconstant amountv of fuel is thus, fed through oricefsand conduit 62 to a fuel feed controlapparatusd .1 cto be described later on. Whenvalvef'l is turned to fully open position, the constant amountofzffuel .corresponds to the needsof Vthe jetfpropulsion engine, at full throttle krunning,'atrnaximum rspeed and in the atmospheric conditions of high speed, low altitude and great cold flights.

(b) Means ,for controlling the fuel feedp The fuel feed control apparatus includes a piston valve 2| arranged for sliding movement in a cylinder 20 and subjected to the action ofspring 2| 1. Provided lengthwise in cylinder. 20 iswan elongated slot 201 through ,thewallthereon the fuel delivery conduit 62 Opening into afunnelshaped member 202 as shown,I which extends throughout the length of slot201. Substantially in the same cross planes vas the ends of slot 201 are ports through the wall of cylinder 20, for delivering fuel into an exhaust lpipe |61 and a fuel main 203. Piston valve 2| as shown has a considerably less axial extent than slot 201 so that passageways for fuel through slot 201 into the cylinder inner spaces on either side of piston valve 2| are available, but it will be readily understood that the rates of fuel supply to said spaces and thence to exhaust pipe |61 and 203.11espectively vary differentially in accordance with theactual position of piston valve 2| in cylinder 20. t

yThe exhaust pipe |61 leads to a portthrough an end portion of a valvecylinder `46 which is a part of a compensatingj `c levicevgenerally referenced 23. y A l Arranged for slidable movement in cylinder 46 is a piston valve 45 adapted to establish or cut-off communication of that spaceofsaid.cylinder to which exhaust pipe |61.leads,.with the fuel return pipe I5 above referred to. Branched off main-203 is a pipe 49 leading to the space in cylinder 46 on the other side of piston 45. v s

Assuming that piston valve 2| isV moved to the left and thus causes a restriction in the rate of fuel delivery to main 203 with va consequent increase of the rate of fuel delivery to `exhaust pipe |81, piston valve l5 is moved to the right and opens the port to return conduit I6 more widely.

Starting from main 203 .towards the fuel injection nozzles or jets 22 is a fuel conduit 65 which branches into two conduits 63 and 64. Conduit 64 is unobstructed and secures a minimum fuel supply to nozzles 22. 1

Inserted in conduit 6s is fa valve.device comprising a valve body 501 in which a fuel passage control piston 5S is biassed to cut-off position by a spring 5| and is further subjected to fuel pressure in the portion of conduit 63 located downstream', through a pipe 502. The object of this valve is to maintain a sufficient pressure of `fuel in conduit 65.

(c) Actuafzng means for the'fuel feed control means.

(l.) CONTROL AS A FUNCTION OF `PRESSURE @message fluid isuch; as oikstheomp'osite .slideivalve'ibing movablycarrangedllm an: open-ended: cylinder-12B fas shown-,'.xpressureloilcis red itocylinder iiirom :a pump;v 3 2 'vial apipee33. Starting fronrcylinder :islficontrolled byevalve relenients? 2.62 'fandlislre- .-spectively; rpipes *.30 .trandal :.ileadifto Uthemnner fspaces) on` eitheri sidefnf Lalfpiston 21 1in .acylinder 56. Fast awi'thpistonz'l .zissazsro'd Which-.Tis coupled ywith a.rod.5'|y fast .with ypiston valve 2|, through Va lever v2'8 pivotally connected with both rods ;`y rotatably supported i on "leverf I zis l-a'`fr"oller :58'2arrangdto1move on arstationary'1surfaeee58n An"=oil discharge pip'eSS-havinga fspring=urged l-eheck valve i160 thereiniis -connecte'dewithf-pipe vfor return2 ofr excess=oil1from=pump 32.

In conduit -iromi cylinderyto fcylinderi @is `inserted-a Yvalve devicegwhich comprisesafslid- Vable member 43 includingitwolspaedapartpisftonsfl, 62,1'said member being tmovablyarranged 'in a cylinder-'1431 pistonsffl 14512# aref adapted'to `-control' the oil -ow' throughcon'dit'l. "The slidable member 43 is -arrangedlto'be'lmoved by'a x'centrifugal 1 governor L"d'2 *driven Lfrom f y"turbine *3, 'againstthe opposingfforce offa SpringHill.

the opposite endfleven 36 is c'onneetedwitha `tem-- -peratureresponsiveicontrolfdevice-toibefdescribed below,- lthe point -vof vv-vzsonta'ct'between" ilever 3S-'and rod 15381 lying -between "said- I'-erids.

(2.) CONTROL As A FUNCTION OF TEMBERATURE The heat-expansible member SAsha-siansintegral 'side arm :onwwhich i is a pivot, pin:r 3 5:' arranged .to m'ove. in:alongitudinalslider3.61 in lever 3.6.

= .The controlr meansidescribed abovezunden'li and 2 Operate". as :follows:

The variations of air. .temperature-iat rtheninlet Y' of "compressor causefdeformatinnsfof :the heat expansible member 34 and .hence displa'cernents ofypivot -pin' v35v for lever 36.

ezinsequeneefthef coritrliim'eans;:causea'onlyifthe fuel amount la` as referred to in the opening state- :mentsfioff-.fthis specification utoltllel combustion -Qmamber. '1n :all f operationf.1conditions, wpis'ton valve 2| and the lower endzo'fsleverhwhihfengages rod 5\'|offsaid,piston walveare shifted to the right to a variable extent. `If"from such a positionppivot pin'i 3'51 lis'y raised asfai fre'su'lt' of# an iincrease in air :temperaturelbefore compressori-il, the point on lever 36 wherelrod'iSf-abutsismovd 1vto the fright# and "if fair pressuref-has' not'- altered fmeanwhileflthat isi ftoesay if-the Aa-iiial@length/if i capsules 53 ,11 EQ-hasremained unaltere'djrodsff 381, 382 A'anfl' l hence J'slide 'valveelementsZ 61-'f-'2 6r are 'shifted to "the r'ight. C'onsequexitlyoil from'pipe 33 is forwarded via the space-rin-c'ylinderbetween valve elements"1262;@2li3intofeoriduit lt'lfand E'thence' into cylinder* 15 6 :pistoni 521y iis thus lmovd to the right,'causing'linl2Sitolswing-fabout'=rdller l58h-and lconsequently v-to-move i'pist'onwalve l to thef-l'eft Vthrough "thelistemi'i theref, Auntil-"tile clockwise swinging of lever 3Gab'out'pivotpinJS'S 'allows rods '3815382 -movedl to thele'ftbylspringf, to'bring -s'lide valve elements'izifZGz tobil-'cutff 7 position.

fWhile Aslide Kvalve lements' 5261-126@ arerm'ovd Vas afresult of pressure-and/ortemperaturealterations, the lvalve device# 43 'linsertd in conditf-Bfl 'from cylinder 2G to eylinderjfv-iisimaintainediby springfll''iin" suchalpos'itiori asto lallow -afi-free passageway for oil through conduit 3|.

(d) `Sinead governor .Thefspeedrgovernor 42.2isza saetydevicawhieh only is effective to move valve device 43 only when the number tofV revolutions. 0fthe motorA exceeds a g'iven'limit. At this time only the speed governormoves valveV deviceffagainstthe opposing 'force of spring- 544, and'the` piston v'alves''l 62 fof- .valvel deviceV 43 cause suenan-'alteration ini-the .flow f of lOil through conduits@ ,'1'3 lffto cylinders-"56 that the displacement` off-pistonZ-'l `:results` '1in' "a restriction of the fuel@ supply through apparatus '|11 to nozzles '22 'Via"condliits-s"and-F54, irrespec- AThevariations .of Lair'pressureattheliinletzrif compressorvl cause deformationsfof pressure cap- 2| sin being :so fmoved, -eauses :lever 36 :ntof swing and consequently .ftozshift rodsf331,:38z rtof. a vposition vvfor lwhichsslide:'valve 1elements'i2B1-e264f: cut off further;A supplyfofioil .to cylinderir.

The position shown on the drawing for Ylevei corresponds ito'lallimitase "(which'lwillfnotoccur inpra'ctiem2\wherein'theairfpressurelbeibre'eompresser AI 'islsubstantillyiequal'ftozero'landrasra -Byf moving 1valve` v'if'tl'xroug'fh' handle- -9- "therefor, the flow through orice 8 and consequently-the feed fto apparatus '-'IfI canbe'redueed, resulting in a decrease of the ypower output-ofA the engine.

T'I'he control means 1 according to y"this invention nan also be-vuse'd as abovestated in'f'the case where fthe'- propulsionimeans -are adjustable.

The control means' above ``:described permit 'to obtainrmany technical-advantages; arid-notably the-following:

"\1."'Ihe Arcontrol'imeans accordingito ftliis''inventionsenable of-"adj listing t-he vfuel supply` as -a un'ctionf'of `the'f-variai'sionsfofain-temperature and pressure :prevailing at -theentranee ofZ compressor that isl all the static lcharacteris'tics rdf'foutsiizle air; -1and at the 'same time the 'altitude "andght speed o'f "the plane lare taken into--aceount*auto mati'clly"and-'accurately *2. fThe' control'means comprisefa-fueldistribu torv` whicht isrsorconstructed *thatdtheamount' of -iuell supplied tothe'irij ection`noz`zle`varies linearly with the "stroke `V of 'the v'adj usting member 'therein. ./.lls 'aronsequenee Jtheairpressurel "and air temperature r*responsive elementswhich 'aotuate Vsaid adjusting `'m'eniioer "through 'a movement amplier may be constructed as simple lements with a linear characteristic, and complicated transmission members such as cams with quadratic contours can be dispensed with.

3. The control means can be easily adapted to the different types of turbojet and turboprop engines and fuel injection nozzles.

4. In many cases, the control means according to this invention permit to employ a fixed exhaust nozzle.

What we claim is:

1. For turboj et and turboprop power units having an air compressor and a fuel burner, the combination of a temperature-responsive device adapted to be exposed to the air on the inlet side of said compressor; a separate pressure-responsive device ada-pted to be exposed to the air on the inlet side of said compressor; a source of fuel; a valve casing having a fuel inlet slot therethrough; a valve supported in said valve casing for movement along said slot, adapted to divide the inner space of said casing into two compartments of variable volumes communicating respectively with the portions of said slot on either side of said valve; means for supplying fuel from said source into said valve casing through said slot; means for conveying fuel from one of said compartments in said valve casing to said burner; means providing an exit for fuel from the other compartment; and means connected with said pressure-responsive device and said temperature-responsive device, for moving said valve in said valve casing according to a predetermined function of air temperature and air pressure variations.

2. For turbojet and turboprop power units having an air compressor and a fuel burner, the combination of a temperature-responsive device adapted to be exposed to the air on the inlet side of said compressor; a. pressure-responsive device adapted to be exposed to the air on the inlet side of said compressor; means for conveying fuel from said source to said burner; means in said fuel conveying means, providing a fuel passage of variable cross-section, for controlling the flow therethrough of the total amount of fuel conveyed to the burner; and an operative connection between said temperatureand pressure-responsive devices and said fuel flow control means, for causing said fuel ow control means to alter said cross-section according to a predetermined function of air temperature and air pressure variations; said fuel conveying means comprising a first, unobstructed fuel pipe from said fuel flow control means adapted to be connected to said burner, a second fuel pipe from said first pipe adapted to be connected to said burner, a biassed-to-closure, throttling valve in said second fuel pipe, and means sensitive to fuel pressure differential upstream and downstream with respect to said throttling valve for controlling said throttling valve.

3. For turbojetv and turboprop power units having an air compressor and a fuel burner, the combination of a source of pressure fuel; means for conveying fuel from said source to said burner, comprising a variable area fuel-metering orice; a temperature-responsive device adapted to be exposed to the air on the inlet side of said compressor; a. separate pressure-responsive device adapted to be exposed to the air on the inlet side of said compressor; a. movable member adapted and arranged to vary the effective area. of said fuel-metering orifice; an operative connection between said temperatureand pressureresponsive devices and said movable member, for so varying the position of said movable member in response to temperature and pressure changes as to keep a fuel flow B through said orifice, approximately defined by the formula wherein is the theoretical amount of fuel delivered to the burner at full throttle run in the case where the pressure at the inlet side of the compressor is normal atmospheric pressure and the air temperature at said inlet is 288 K., pi is the pressure at the inlet side of the compressor, 1- is a constant, Ti is the temperature at said inlet side measured in degrees K., and b is the amount of unburnt fuel; and means for discharging fuel from said fuel conveying means in response to fuel pressure differential across said fuel metering orifice.

4. The combination of claim 3, further comprising a flow restricting device in said fuel conveying means between said fuel pressure source and said fuel metering means, comprising a hand-controlled valve; and means for discharging fuel from said fuel conveying means to said fuel metering orifice in response to fuel pressure differential across said iiow restricting device.

5. The combination of claim 4, the last-named means comprising means providing an exit for fuel at a point in said fuel conveying means between said source and said hand-controlled valve. and means in said exit means sensitive to fuel pressure differential in said fuel conveying means upstream and downstream with respect to said hand-controlled valve, for controlling the discharge of fuel through said eXit means.

6. The combination of claim 3, said pressureresponsive device comprising an evacuated capsule so as to be substantially irresponsive to air temperature variations.

'7. The combination of claim 3, said fuel conveying means comprising a pair of fuel pipes to receive the fuel eiiiuent from said fuel-metering orifice and convey the same to said burner, one of said pipes being unobstructed; a biassed-toclosure, throttling valve in the other fuel pipe; and means sensitive to fuel pressure dierential upstream and downstream with respect to said throttling valve for controlling the same.

HERMANN OESTRICH. SIEGFRIED DECHER. WOLFGANG STEIN.

References Cited in the file of this patent UNITED STATES PATENTS Number Name Date 2,305,070 Butler et al Dec. 15, 1942 2,422,808 Stokes June 24, 1947 2,426,740 Mock Sept. 2, 1947 2,457,595 Orr Dec. 28, 1948 2,531,780 Mock Nov. 28, 1950 FOREIGN PATENTS Number Country Date 603,884 Great Britain June 24, 1948 

